Page 36 - Fiber Bragg Gratings

2.2. Defects in glass 17

easily changed to arrive at a nearly complete chemical reaction, depositing
a mixture of germanium and silicon dioxides. It is not possible to have a
100% reaction, so the deposited chemicals have a proportion of suboxides
and defects within the glass matrix. With sintering and preform collapse,
these reaction components remain, although further alterations may take
place while the fiber is being drawn, when bonds can break [11-13]. The
end result is a material that is highly inhomogeneous on a microscopic
scale with little or no order beyond the range of a few molecular distances.
The fabrication process also allows other higher-order ring structures [14]
to form, complicating the picture yet further. There is a possibility of
incorporating not only a strained structure, but also one which has ran-
domly distributed broken bonds and trapped defects.
This is especially true of a fiber with the core dopant germanium,
which readily forms suboxides as GeO^ (x = 1 to 4), creating a range of
defects in the tetrahedral matrix of the silica host glass. Given this rich
environment of imperfection, it is surprising that state-of-the-art germa-
nia-doped silica fiber has extremely good properties—low loss and high
optical damage threshold—and is a result of better understanding of
defects, which lead to increased attenuation in the transmission windows
of interest.
Among the well-known defects formed in the germania-doped silica
core are the paramagnetic Ge(ra) defects, where n refers to the number
of next-nearest-neighbor Ge/Si atoms surrounding a germanium ion with
an associated unsatisfied single electron, first pointed out by Friebele et
al. [17]. These defects are shown schematically in Fig. 2.1. The Ge(l) and
Ge(2) have been identified as trapped-electron centers [18]. The GeE',
previously known as the Ge(0) and the Ge(3) centers, which is common
in oxygen-deficient germania, is a hole trapped next to a germanium at
an oxygen vacancy [19] and has been shown to be independent of the
number of next-neighbor Ge sites. Here an oxygen atom is missing from
the tetrahedron, while the germania atom has an extra electron as a
dangling bond. The extra electron distorts the molecule of germania as
shown in Fig. 2.2.
The GeO defect, shown in Fig. 2.2 (LHS), has a germanium atom
coordinated with another Si or Ge atom. This bond has the characteristic
240-nm absorption peak that is observed in many germanium-doped pho-
tosensitive optical fibers [21]. On UV illumination, the bond readily
breaks, creating the GeE' center. It is thought that the electron from the
GeE' center is liberated and is free to move within the glass matrix via
hopping or tunneling, or by two-photon excitation into the conduction